@Sam Williams
There are two issues related to mortgage lenders if your end-buyer is using a mortgage. Neither FHA nor conventional lenders will allow their client to purchase via assignment, so you have to take title in order to sale the property to your buyer. The second issue is one of "title seasoning". FHA does not currently have a title seasoning requirement (they used to) but your sale contract has to be dated after you are in title and the appraisal also cannot be ordered before that time, so you will need to go into title for 1-4 weeks (however long it takes that lender to underwrite and close). If your resale price is greater than 20% of your purchase price two appraisals will be required, which typically increases the time line to close. I have funded a same-day FHA deal but that was a few years ago and have not seen one happen since then.
If the buyer is using a conventional mortgage most all lenders have a 90 day title seasoning requirement (some will be longer such as the 6 month time line you heard of). A portfolio lender that underwrites their own deals may not have title seasoning, but they are somewhat rare after the crash in 2008. There is one exception you want to be aware of because most loan officers do not know that it is in their underwriting guidelines, but if by chance your buyer is putting 20% down the title seasoning requirement can be waived. It is possible to close this scenario the same day but it will be a challenge finding a lender that can or will jump through that hoop.
Your greatest chance of success is to fully disclose up front and make sure the lender understands that the property is being purchased and immediately resold. I suggest not taking the response from a loan officer but insisting that the scenario be presented to an underwriter.
Cash is King if you can find that buyer. Good luck.