Article 1261 of the Spanish Civil Code states that for a valid contract to exist three basic criteria must be satisfied.  First, the parties must be in clear agreement; second the subject matter of the contract must be certain; and third the purpose of the contract must clear.

There can be no agreement if the parties have reached their agreement through a misunderstanding as to any of these basic three criteria, or if they have been subjected to any violence, intimidation, or deceit (Civil Code Article 1265).

Misunderstandings often arise because the parties to the contract have not expressed themselves clearly, and in this case the words that they have used are to be given their literal meaning (Civil Code Article 1281).

If there is any ambiguity in a contract then it will be interpreted according to what is customary in the country, the country for Spanish business being Spain (Civil Code Article 1287).

So, if there is no specific agreement on using another currency it is assumed that any sum to be paid under a contract which is to be performed in Spain will be paid in euros.  The euro was adopted as the official currency of Spain by law 46/1998 pursuant to Article 149 paragraph 11 of the Spanish Constitution, which grants the Spanish government the power to regulate the monetary system.

Incidentally, the maximum number of coins that are legal tender for any transaction is fifty, of any denomination.

If you do intend to enter into any dealings in a currency other than the euro you must be clear as to which currency it is to be, you must fix the amount either exactly or by reference to a an exchange rate, and when, where, and how it is to be paid.  

Exchange rate fluctuations can have an a significant effect on the cost or benefit to the parties involved in a transaction and for a large amount you would be well advised to take advice from your bank or a recognized foreign exchange dealer to minimize the risk on either side.

Remember that regulations to prevent money laundering make it impossible to deal in large sums in cash, and that completion of any property transaction in Spain must be formalized by a public deed prepared by a Notary (Civil Code Article 1280) and the Notary must be satisfied that the money has come from a legitimate source and that it has in fact been paid by the buyer to the seller.  

Do take advice on getting these matters clear at the beginning of any transaction involving foreign currency so as to avoid misunderstandings, maintain amicable relations between the parties, and reduce the risk of the deal falling through.