Cash transactions for money lender

Yes allowed but it's better to bank to bank if such transactions arises as it will be good for future

Any limit for cash transactions to customer amount pay

If a person is doing business or in proffession then a daily limit of 10000 in cash payment to a single person beyond it then he or she cannot get any deductions on the expenses as per incometax
to a transporter you can pay 35000 in cash in a day
when you repay or accepting loan or money then 20000
deduction under 80D upto 5000 in cash
donation upto 2000
as per the section 269ST no person shall receive an amount of 2 lakh or more in a day be in bank to bank or cash

Assess un secured loan from one person Rs:19,000/- + int 1900 total 20,900 Rs:9000 Rs:9000 Rs:1900 different dates loan repayment cash mode accepted

In my opinion accepted but again I say it's better to do bank transaction specially for unsecured loan or any loan

No, you can not do