subscription deleted by admin

The point is:
-For now we do not use any online payment (we will in the future)
-I have set the system to User Approval by Moderator "before the paiement"

the procedure is:
-the user ask for a plan
-we give him the access (in the comming days, up to 60)
-than he needs to pay (in 15days)

So how come the basket is all ready "pending"?
if the user isn't confirmed yet, than the paiement process shouldn't be started?

You should have an offline payment gateway in that case. After they click the payment button it should take them to the payment slip page where they have a link to click to confirm. Once they confirm it'll go into a pending state, which should not expire like a non-initialized basket. More than likely you've users not clicking the confirmation link on the payment slip.