Offer accepted, adding contingency to P&S?

[Max K.]

My partner and I recently put a cash offer on a fix & flip property that we plan to finance with hard money. The offer was submitted and accepted without any contingencies. We already have a letter stating pre-approval from the lender for 90% of the purchase price plus rehab costs, however, there is always a chance (although slight) that the lender falls through. We are going to execute P&S later this week but have some hesitations in doing so without any contingency subject to financing. Our thought is that we may add this to protect our 10% deposit should we not be approved. My understanding is that this is highly unusual to change the terms of an offer as you move to P&S, but it's something that was overlooked when we submitted our offer, and we're not sure that it's a chance that we want to take. So we can protect our deposit, but this could spook the seller and cause them to walk away. Thoughts? Any additional pros and cons of doing so? What am I missing? Thank you!