From the Regulation of Personal Income Tax:

The following concepts are considered work income, and therefore subject to retention:

Diets and allowances for travel expenses earned in a place other than the regular work of the recipient, with the following exceptions:

• The locomotion expenses, in the following conditions and amounts:
  • When the employee or worker uses means of public transport, the amount of the expense that is justified by invoice or equivalent document.
  • In another case, the amount that results from computing 0.19 euros per kilometer traveled plus the toll and parking expenses that are justified.
• The normal expenses of maintenance and stay in hotels, restaurants and other catering establishments in the following conditions and amounts:
  • When you have spent the night in a municipality other than the usual place of work and that which constitutes the residence of the preceptor:
    • For expenses of stay, the amounts that are justified.
    • For living expenses, 53.34 euros per day, if they correspond to travel within Spanish territory, or 91.35 euros per day if they correspond to travel to foreign territory.
• When you have not spent the night in a municipality other than the usual place of work and that which constitutes the residence of the recipient, the allowances for living expenses that do not exceed 26.67 or 48.08 euros per day, depending on whether you are traveling within Spanish territory or abroad, respectively.

This accreditation may be done, according to the article 106 of General Tax Law (LGT) by any means of proof admitted by law. We can point out the following for information: ...

• Company listing detailing day, place and reason.
• Certificates issued by the companies visited.
• Restaurant bills in the worker's name.
• Combination of some of the above.

The various methods indicated, in this case, could, in principle, serve. However, the proof, which will be extended to the existence of the payments and their connection with the actions indicated, will correspond to the management and inspection bodies of the Tax Administration.

But it must be made clear that ONLY FOR EMPLOYED WORKERS THERE IS A SPECIFIC REGULATION OF INCOME EXCEPT FOR LIABILITY that refers to the expenses of diets and locomotion.

In other words, if I work as a self-employed worker and the client company makes me go from Alcobendas to Alcalá de Henares, I would initially be left out of the previous regulation, since the self-employed workers calculate, in the IRPF, the income from the economic activity by the systems legally. established (direct, normal or simplified estimation, and objective estimation).

The most correct thing in these cases is for the self-employed worker to pay the locomotion expenses and then pass them on to the company as the greater amount of the invoice receivable. The company can not justify the payment of allowances if it does not have workers on payroll, so the amount paid to a professional for transport costs is considered a greater amount of the services provided and is included in the invoice. In this case, the expense would still be tax deductible for the company since it would be expenses for services of independent professionals (being transportation expenses included in the invoice) and for the self-employed worker it could be a deductible expense in calculating the performance of the economic activity in the IRPF if it meets a series of requirements.

In these cases, the provisions of article 26.1 of the Income Tax Law are followed, as well as in article 10 of Law 43/1995, of December 27, of the IS, where in accordance with the principle of income and expense correlation , the expenses for professional trips will be considered fiscally deductible when they come demanded by the development of the activity, provided that, in addition, they fulfill the other legal and regulatory requirements.
This correlation must be proved by any of the means generally admitted by law, being the competence of the verification and inspection services the assessment of the evidence provided. In the event that there is no link or it is not sufficiently proven, these travel expenses can not be considered fiscally deductible from the professional activity.
In the assessment of this correlation, the individualized amount of the travel expenses must also be taken into account, so that those that are excessive or excessive, with respect to those that, according to the uses and customs, can be considered normal expenses, can be considered as non-deductible, since these are border concepts between the expenses demanded by the development of the activity and those that come to meet particular needs, in such a way that in some cases there is a coincidence between them. It must not be forgotten that the tax regulations (articles 27 and 21 of the Law) require a distinction between the patrimony of the taxpayer affected by economic activity and that destined to the attention of particular needs, and that the tax regulations themselves establish maximum amounts exempted from tax in the case, for example, of expenses of maintenance when it comes to expenses produced by labor displacements, through the regime of exempt diets that before mentioned to you (Article 8. Tax Regime).

Therefore you can deduct such expenses in the income statement to the extent that it is understood that they are related to obtaining the income. You can use as a reference to determine this correlation the locomotion expenses data of salaried workers (€ 0.19 per KM, etc.).
In the case of the diets, if we receive 50 euros but we only have restaurant bills for 30 euros. We must declare as income 50 and as expenses 30.