Ian Ziering's willing to do a little horse-trading in his divorce ... he says he'll help out his estranged wife financially for a bit, but then she's gotta get a job.

The '90210' star just filed a response in his divorce case and, according to docs, Ian says he's down to pay Erin Ziering "reasonable spousal support for a limited period of time," but he's also requesting "an order that [Erin] become self-supporting."

In docs, obtained by TMZ, Ian's also asking for joint legal and physical custody of their 2 minor daughters. Erin has asked for sole physical custody when she filed divorce papers. Translation -- it's acrimonious.

There a discrepancy on the date of separation -- Erin listed October 31 while Ian listed October 14. As we reported ... Ian announced back in October he's splitting from Erin after 9 and a half years of marriage.