Cash from seller after close on 1031 Exchange

[Isaac S.]

Hi, i have a 400K 1031 - and i identified 3 properties. In making offers, i found that the properties although listed above 400k, will probably after due diligence come in at 350K. If i sign a contract at 400k (to satisfy the 1031 req), but say the seller pays me 50K at close, i'd imagine that's not allowed. 

If it's not allowed, is there anything else i could do in my scenario instead of just over paying?