I suppose, the problem is caused by the (relatively) new feature of monthly invoices. Wthout monthly invoice, you would already have an open invoice whch you could pay immediately and unblock the transfer. But now, there is no invoice yet, just one or more open positions, which will appear in the next monthly invoice. And you don't have a possibility to pay these open positions now, at least no obvious one.
This is definitely not good. Either the open positions, which don't appear on an invoice yet, should not block a transfer, or there should be a possibility to pay these open positions and unblock the transfer. As it is, open positions for a product have always to be paid by the old customer. Therefor, the first alternative will likely not be acceptable for netcup.
So a nice button to generate an invoice for (all) the open positions might be a possible solution. This invoice, once generated, would then have to be paid within the usual 14 days and its generation would then empty the preview for the next monthly invoice. Once this invoice is paid, the transfer would be unblocked until the next open position shows up in the preview. So this invoice still might need to be paid really fast in order to unblock the transfer, but this would at least be possible using Paypal, Credit Card or whatever is working instantly. Better than waiting some more weeks with the transfer. Just a few thoughts, though ...