ESA and permitted work payments

Please can anyone help with this as I cant even get a straight answer from ESA

I am in the support group contribution new style esa - I am self employed and can occasionally health permitting do some work (which I have informed them about) as I really want to try going back to work - I am an artist

The problem is my work is commission based and I get paid this at the end of my job - so even tho I would only be able to work a few hrs a day and probably only every few days (which wouldn't take me over the allowance re payment) - I would get a lump sum at the end.

for example

I may work 6-8 hrs a week earning - £120-£140 a week - the job may take me 6 weeks so I would then get £840

This would take me over the monthly allowance on esa but this is actually for 6 weeks work - how do I show this ? will they stop my esa because of this lump sum ?

I just don't get how I can do this - I fill out my income and expenses on my UC

any advice appreciated