What the Treasury seem to be saying here is that they're uneasy about handing over borrowing powers to the Welsh Government when their only substantial means of paying it back is the income stream from the block grant - they'd like them to have the paper round as well as the pocket money.

BBC: Pocket money and paper rounds

That would give the Welsh Government that all-important revenue stream to borrow against independent of the block grant.

BBC: Ministers, elephants, and carrots