This really isn't a math question - it seems more like an accounting question. But I'll try:
If the original aim of the sinking fund is to have a pool of $3K that can be used by the corporation, and if each partner is contributing an equal amount, then each partnet must contribute $1000. Now if one person wants to contribute $400 cash + $600 in services that would be OK only if the $600 donation of services is truly a savings to the corporation. In other words by contributing $600 in services (washing windows or whatever) does the corporation save $600 in cash expenditures that it otherwise would have had to pay? If so - then the corp could accept the $600 in services and transfer $600 out of the expense budget and into the sinking fund. End result is the sinking fund has the $3K in cash it needs, and the expense budget took a $600 hit for expenses it was planning on anyways.