What happens when i buy a land and was only issued a letter showing i paid for the land

First a letter in form of a “receipt” showing payment for land is proof of transaction, but not proof of ownership unless it’s backed by a Deed. so, the receipt alone proves that money exchanged hands but does not automatically transfer ownership especially if the seller has no right to sell.

Secondly, if the land belonged solely to your late uncle, and he did not will it to his wife or it was family land under customary law, his wife had no legal authority to sell. Her sale to you would be void or at best voidable, meaning it can be challenged by the family or rightful owner (if it is the uncle who shot).

Thirdly, because you sold the land, you’re now in the position of a vendor (seller). If your initial purchase wasn’t valid, the subsequent sale can also be challenged as invalid. This means that you might have sold land that legally wasn’t yours to sell. However, possession over 7 years can sometimes strengthen your claim (if you have occupied or been known to have had the land openly without challenge). 

finally, gather all your documents and witnesses and consult a legal practitioner especially one experienced in customary land disputes. your matter is quite complex, which means you will need proper legal consultation to guide you on the right steps to take.