Is the cash payment invoice from our contractor legally valid?

Questioner

We have received a cash payment invoice with a date from a contractor for an amount of just over 10,000 euros. I would like to know if this invoice legally confirms that we have paid a cash payment to this person/company. The contractor does not show up to complete the work, which means we have now lost money.

Lawyer

Do you have proof of payment? For example, a signed proof of payment, or perhaps there were witnesses to the payment? The contractor should be summoned (by registered mail) to commence/complete the agreed work. A reasonable period of time must be given. If the summons is not complied with, certain rights may arise, such as the right to terminate or to enforce compliance through the courts.

Questioner

The invoice only states payment by cash with the date but no further signatures or anything and the only witnesses are us and the contractor :(