Update Record If Unique Match

This is probably dead simple, but I am brain dead today.

I have two tables:
Requests with fields (ID, Cust, Amount, Ref, Date)

and

Actuals with fiels (ID, Cust, Amount, Ref, Date)

Now,
if Requests.Ref is null, then update Requests.Ref = Actuals.Ref and Requests.Date = Actuals.Date if and only if
there is only one record in Requests and only one record in Actuals where Requests.Cust = Actuals.Cust and Requests.Amount = Actuals.Amount.

Currently, I am just doing an inner join between the two tables, but if there are two requests with a given cust/Amount, but only one Actual, then both Requests will get the same Actual Ref and Date.

So how do I structure this SQL?

Thanks,
David