Proof of Funds Help Needed

[Aaron Hollingshead]

I came across a deal that looks too good to pass up and made an all cash offer as it can't be financed.  Problem is that most of my cash is tied up in a flip property which is set to close in 2-3 weeks at which point I'll have enough cash for the purchase and renovations of the new house.  

If my offer is accepted, I expect they will want proof of funds within a couple of days.  I don't want the offer to be contingent on the closing of my other place as I think that this property will have multiple offers and I don't want mine to look weak.  I was wondering if showing sufficient cash reserves in index funds can work for proof (even though I don't intend on using this money)?  I would be able to present about 40% of the cash needed in my account and the other 60% in index funds.  Any advice for another creative work-around?