There's no legal standpoint unfortunately, unless you have receipts, or otherwise want to go to small claims court and attempt to prove Lyndsey had ownership somehow.
We used to give dogs out before donations had been paid, they'd signed contracts to say the dog legally belongs to Huskies in Need for the rest of it's life etc. but when people didn't pay and we tried to pressure them to either pay or give the dog back, we've discovered that we'd have to take them to court at high cost and hope we win - which isn't guaranteed! So with no paperwork at all I doubt you'd have any chance really. Needless to say we have to be really strict now and get donations from people before dogs go out.
On that note, why not just adopt another dog from rescue for the granddaughter?