This varies from city to city in Miami-Dade Florida you can back out of a condo purchase within a certain time frame 3days for new and 15 days for a resale. In your case you indicated that the buyer had a option to back out after closing you do not need a lawyer to tell you that if you have an option you can exercise it.
However that option if done right should state what will be done with buyers credit and basically how everything will be handle to permit a reasonable smooth "backing out" if that was not done then it could get real complicated, in which case you will need on attorney, if you and the seller cannot come to on agreement on how to resolve the matter amicably.
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If due to some reason buyer wants to back out of sale after closing, he would need to consult with the attorney to discuss his options or any grounds, if any.
It all depends on how your contract was written and only an attorney is qualified to interpret that contract.
Hope this was helpful.