Hi bhambulls,
George did a better job of explaining that than me (thanks George!).
Some lawyers will advise that, if buying a property at significantly less than market value, you pay the Complementary Tax upfront (some time last year, I did a calculation for someone whose lawyer had recommended paying it upfront and I was able to demonstrate that, in that case, he would still save money - for a sale price of €95k against a €115k selling price). However, some buyers think it's worth the risk, not pay it and sit out the 4 years in the hope that the tax office don't apply it. If the tax office does apply it, they may also apply interest and that's where it could become marginal. Avoiding payment - and I'm not suggesting you would - takes you into a completely different level of risk: penalties applied by the Spanish tax office are punitive, to say the least, and could make any Complementary Tax levied look like small change.
Hopefully, your lawyer is independent and not tied to the selling agent. If not, you should seek independent advice.
Kind regards,
Kim