Sale deed objection made by seller the she was not mentally fit at the time of sale

is the original owner of the property not signed the sale deed?  "The sale is for the purpose of loan repayment of the seller's husband" was written in the document, then why did you not ask the reason why the wife sells her self property for the purpose of her husband's loan?  The document has to be verified.  So consult a local lawyer with the material/papers you have.

The property was of the husband after the death of husband wife sold it for the payment of husbands medical bills. The husband died due to illness after hospitalized for about 3 months.

The facts posted are not clear but confusing and vague, it would be appropriate to consult a local prudent lawyer with relevant documents.

if the property was in the name of seller and duly registerd before court of sub registrara ,then that is not easy to throw challenge to such sale .

You must be sure - that the cheque is received back and not with seller or in the recipt of payment it is mentioned that the payment is against chequ number for sale of such and such property.

That  propertyw as in the name of seller and the sale deed was duly signed by her.

The sale deed is registered before court of sub- registrar and she appeared herself before registration court.

In the above circumstances there is no need to worry