Does Ought Imply Can?

I was having a conversation with my friend Mike Morrissey that culminated into the following counterexample. I don't know if similar ones already exist, but this is what I thought of last night.

First, what does

(M) Ought implies can.

mean? I take it to be the case that if a person P has an obligation to do A then P ought to do A such that if P doesn't do A then P is morally responsible for not doing A. "P can do A at time t" just means that it is within P's power to do A at t, or that P has the ability to do A at t.  So I take it that the following follows from (M):

(M*) P ought to A at t only if P can do A at t.

Moreover, P can do A at t only if it is logically and physically possible that P does A at t. So if it is physically impossible for P to do A at t then it is false that P can do A at t and, therefore, it is false that P ought to do A at t. Consider the following counterexample to (M).

Suppose P signs a contract willingly. Under the contract, P must pay her rent on a certain time. Given that P signed it willingly, knowing all of the details, including the time at which she must make the payment, she has an obligation to pay the rent at the stipulated time. So P has an obligation to pay rent at a time t2. However, P knowingly and deliberately did not come up with the necessary funds in order to fulfill her obligations. Now let t1 be the millisecond before the time at which she must make her payment. At t1 it is true that P has an obligation to pay her rent at t2 and it is true that P can't pay her rent, for it is physically impossible for her to come up with the funds at t1 given a lack of time. So it is true that P ought to pay her rent and it is false that P can pay her rent. This just is the negation of (M).

This is just a quick reaction and I haven't looked into the relevant literature. So it might well be the case that I am wrong, but this looks prima facie sound.