I have sold several properties at under escritura price and it can be done.
Firstly the selling lawyer should go to hacienda and inform them of this intention so they have a record of it.
Then all monies handed over have to be shown and preferably in cheque format
All purchase contracts lodged with the notary as well.
We are still warned that hacienda may try to levy the difference in the future but also remember the escritura price is more than the catastral price as well.
So far we havent had any problems but I will let you know if there are.
The first time we experienced this was 3 years ago.