Via The Points Guy, Delta and American Express are effectively offering a 40% rebate on Membership Rewards points transfers through September 30th (registration required).

For every 50,000 points you transfer from Amex to Delta Skymiles, you’ll receive an electronic certificate to deposit 20,000 Amex points back into your account.

Only 50,000 mile increments count, so if you transfer smaller amounts that total more than 50,000 points you’re out of luck. A 60,000 mile transfer and a 90,000 mile transfer won’t get (3) 20,000 point rebates, just two.

The rebate doesn’t come right away, but rather 6-8 weeks “following registration in promotion” (which I assume is an error in the terms and conditions, and that they actually mean either 6-8 weeks from making the transfer or 6-8 weeks from the end of the promotion).

That means you don’t get any bonus right away. You need to have the full Amex point balance in your account now if you want to make a transfer and use the points right away. You’ll get the points back later.

This is actually a very rich bonus, since it means that a net of 30,000 Amex points yields 50,000 Delta miles. That’s a 67% effective transfer bonus — the largest that I’ve seen outside of the mistakes that Amex and Delta have made in the past (such as the 80% transfer bonus that some members were eligible for in January and the 150% bonus which didn’t include Amex and wasn’t actually supposed to get launched).

The nice thing is that all members are open to this not, not just those doing a transfer from Amex to Delta for the first time as with the current 50% bonus offer which also awards 25,000 elite qualifying miles for making a transfer of 100,000 points or more.

Still, for most members I wouldn’t suggest jumping on the offer. It really is going to depend on your award goals. I tremendously value the flexibility of Amex points, they let you decide where to put your miles later, once you know where you are going to need them — generally because you know the award you’re going to book and who has the availability you need. You’re giving up tremendous option value when you make a speculative transfer ahead of an actual award booking you need to make.

Now, if you need to make a booking during the promotion period it makes Delta miles incredibly cheap. Say for instance you wanted two business class awards from Houston to Europe, you could likely do it via Amsterdam on KLM and instead of being 100k points apiece it would cost you just 60,000 points per ticket (net). You’d transfer 200k Amex points to Delta, book your tickets, and get 80,000 points back later. Not bad.

Again, I’d look very closely for Delta award availability during this promotion if I were using Amex points for the booking. If I could make the award work with Delta that would likely save me some points.

If on the other hand you live in Washington DC and want to fly Air France, you may want to build up your Delta miles even speculatively, there are just so many darned Air France business class seats DC to Paris (now that they’ve added an A380 to the route in particular). Or if you wanted Los Angeles – Bribane in business class on V Australia, though even there I am a little bit skittish because that’s one of those routes where the inventory is just too good, it tells me that either it won’t last or Delta will start restricting access to it. That’s another reason I love keeping points in Amex Membership Rewards until I need them, I’m less exposed to the devaluations which go on across the various programs.

Still, a 40% rebate which is equivalent to a 67% bonus is a tempting thing, and I pass it along for your consideration. The bonus sorta makes up for the fact that you are getting Skypesos, after all!