I've used Cash account to handle something like this in the past. If it is something simple as
On Jan 01, 2021 I've lent Bob $100. I've wrote him a check from my checking account
* In my checking account, transfer $100 to "Lending Cash" (paid to Bob)
On Feb 01, 2021, Bob paid back $50, and I deposit his check to checking account
* transfer $50 from "Lending Cash" to saving account (paid by Bob)
You can use a general "Lending Cash" or create one specific for Bob
For the latter, you can see the current balance Bob Landing Cash is $50