Whilst I don’t hate your being a middleman here, this is one of those that you need to chalk up to your lack of experience and be clear on in future.
I don’t know about your contract and I’m not in a position to advise, but from an ethical point of view, you should take the lesser amount if there’s any confusion and clarify that contractually, because:
a) The confusion is your fault.
b) Regardless of the contract, if you claim that 20% includes the publisher’s share, the band won’t hear that – they’ll hear that they signed up for 80% and now they’re getting 60%.