Back pay?

mia100max

Hi, 
I failed my last esa tribunal so took it to tribunal after my doctor encouraged me. When I was receiving my esa, I was paying £3 a week for an income support overpayment from 22 years previously lol. 
I won the tribunal & was sent a letter saying my back dated esa which amounted to £600 was being kept as payment towards the income support overpayment. I was really relying on that back pay as I had been loaning off friends & family to live & feed myself. Are they allowed to do this. If they had assessed me properly in the 1st place, I shouldn't have been taken off esa. Also, why didn't they just take the  accrued £3 a week towards the overpayment from the backdated esa payment? I hope someone can help me with this. Thanks.