Or,the new owner could offer to sell the piece of land to the Spanish guy ,if indeed it's worth it to him to buy it.The land in question remains on the new owners escritura and is high lighted on said paperwork but the neighbour uses it on a permanent lease.We did exactly that with our nieghbour and all is rosy in the garden.....
There is a price per metre for agricultural land but if the nieghbour really wants it he'll pay.
Check everything with your solicitor beforehand though.