If they have 10BTC or any amount in their wallet on a certain address, if they sign a message using that address we can verify it belongs to them 
You can test it out using your own wallet and sign messages etc.
You can test it out using your own wallet and sign messages etc.
Thats a very good idea but how will OP prove it is really his wallet?
Maybe some trusted member should open some wallet and ask OP to deposit like 1-2 cent to the said wallet to proof it is him wallet and he have the coins that he whant to lend
Maybe some trusted member should open some wallet and ask OP to deposit like 1-2 cent to the said wallet to proof it is him wallet and he have the coins that he whant to lend