You can ask to renegotiate, but have no right to do so (i.e. the buyer can simply refuse) as long as the buyers are honoring all their important ("material") obligations: whether or not the market has changed or you should have asked for more, you accepted an offer, entered into a contract, and therefore are bound to it. Only if the buyers breach their obligations under the contract would you have leverage, since a material breach by one party gives the other party the right to terminate the contract if he/she chooses--so if they breach, you could ask for changes in the terms and if you don't get them, terminate the agreement. Material breaches by the buyer are, however, mostly limited to not paying what they should when they should, so if they have paid everything to date they should and can close on time, you will not be able to do this.