Legal Guardian but Signed Sale deed as "Constituted Attorney"

I have made an agreement to purchase a resale house and paid non refundable 2 lac rupees and got 3 months time to get my home loan sanctioned(In the agreement it is mentioned owner should take the responsibility if any legal problem arises with the property). I have done a verification(By a lawyer) for the ownership of the house and got the documents (Sale deed of Current owner, Link deed, Porcha, Mutation certificate, khajna document, tax receipt) and looking at that initially SBI lawyer told us(verbally) that it will be okay as the dag and khatiyan number shows the name of previous owner(Updating current owners name might take time). Current owner bought the house from a lady(Previous owner) whose husband bought it from another person. Now after the husband died, his wife(Previous owner) sold it to the current owner. She had one son and one daughter(minor is mentioned in the sale deed). Now, The lady and her son signed on the sale deed but her daughter didn't and her mother signed on her behalf. The words "minor" and "natural guardian" is mentioned in the document but, When the lady signed she signed as "<name> as for self and constituted attorney for <daughter's name>" Now the SBI lawyer is calling me and telling that the Sale deed is not valid as she did not signed as legal guardian, but as constituted attorney and there is no document to prove POA. I asked the current owner for it, he also doesn't have it and don't have contact with the previous owner. How should I get out of this trouble? I need to buy the house as otherwise the 2 lac rupees will be forfeited.