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David Geffen 's estranged husband says the mogul's trying to cheap out on spousal support ... alleging David's only offering a fraction of what he spent in a month while they were married.

David Armstrong -- also known as Donovan Michaels -- filed documents Thursday in which he claims Geffen's trying to "aggressively limit" Armstrong's access to temporary and permanent spousal support. Getty

Armstrong objects to Geffen's request that he only pay spousal support for a few more months. He points out David's worth a whole lot of dough -- $8.9 billion according to Forbes -- and Armstrong says he has firsthand knowledge that Geffen still works as a "venture capitalist, high-level investor, and power broker" despite Geffen's claims that he's now retired.

It all comes down to d

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