i am worried that CHild Support will tell me to give it back to him, not the employer.

because, his PAYE slip from that three months of employment, will show a very low number, not the salary that CSA have in the assessment for that period. So i'm worried they will retrospectively work it out, that he should have paid ZERO for those three months, not the amount he did pay. And then they will want me to pay back that three months of CS, to them, to be given back to X.

(he has DD for two nights per week, so even on unemployment benefits, the assessment would be zero).

i think he found being unemployed so dispiriting, that doing the commission work, living on loans, appealed more than living on unemployment and being rejected for ten jobs every fortnight. Like a break, from the constant "you're too old, we prefer the younger applicant". Never said, but pretty obvious.