Just to add to macgdo16 reply.
The amount showing on your escritura was the White money and was normally paid by bank transfer, bankers draft etc.
The difference is the Black money and HAD to be CASH. Most notarias even kept Bank note counting machines for the purpose; but the Notary would usually leave the office whilst buyer and seller completed the Black transaction.
This question has appeared on SPI many times previously and I would normally say; with all due respect and not wishing to cause offence; it may be selective memory.
This practice helped the selle reduce capital gains tax and also helped the buyer pay less purchase tax.