The Agencia Tributaria (Hacienda) keeps a close eye on bank transfers, loans and donations to family members
Banks have to keep them informed of any suspicious transfers made or received, but also covers large amounts of cash deposited. Failure to do so on the banks part would mean certain retribution from Hacienda.

When, for instance, loans or purchases of 6,000 euros and over are made, Hacienda is informed. Any transaction equal or superior to 10,000 euros is also reported.
The thing is, people make transfers to family members and any such movement is subjected to the Sucesiones y Donaciones, and therefore a percentage must be paid to Hacienda. Now, we’re not talking about 500 euros or something similar but anything over 6,000 euros. Below 6,000 euros and there is no problem.
Why the strange figure of 6,000 euros? Because it is equivalent to one-million pesetas and nobody has bothered to change it but instead just convert it to euros.
When we’re dealing with a loan, its the Impuesto de Transmisiones Patrimoniales. The recipient must include it in his or her tax declaration (Model 600) within 30 days. However, no payment to Hacienda is required.
(News: Spain)