If you signed a contract to pay amount X and were only charged "X less Y" then in the absence of a compelling reason to the contrary you will be on the hook to pay Y.
If this ever became a 'legal argument', they will point to the contract you signed, which is pretty conclusive of the price you agreed to pay. Your counter-argument that you're not obliged to pay would be saying they had waived their right to the payment. I personally don't think that argument is compelling because it makes no sense that they would knowingly and willingly do this, combined with your clear signed agreement to pay a different price. From the info that has been presented, the undercharge is most likely to be considered a mistake.
However, the details of your exchanges, including the form of the emails / invoices, may shift the position. I may have missed where this was clarified, but were you issued an invoice that was different from the payment request? What does that say? A request / link for payment is not necessarily an invoice. If you have fully paid an invoice, then that invoice should be settled, but that won't stop them issuing an invoice for the outstanding amount, in which case their legal action will be in respect of that second invoice.
Whether they can be bothered to pursue it is another matter entirely. There may be no desire from their finance team to consider it further if they are sitting on a fully paid invoice (albeit incorrectly issued), so it may just be the one salesman wondering what to do about it.
If it were me personally, I'd willingly pay it because I'd think really poorly of myself if I didn't pay it, but also for peace of mind. But for the above reasons, you may have scope to sit on it a bit longer, to see if it escalates or not, taking a risk on whether they decide to take action on it - if they do take action and you continue to sit on it, you will potentially risk interest (depending on the invoice situation) and paying legal costs.
Edit: actually, I've never heard of 'signing via a Microsoft Teams call'. How was this 'signed'? I won't be bothered to comment in more detail on this topic, but perhaps there is something in that which collapses the concept of there being a signed contract, which changes the evidential burden. Maybe so, maybe not. But you've already admitted on here the price you agreed to... so there's that as well I guess.
Actually a dangerous thing to do - by then acknowledging the debt it potentially gives them grounds for pursuing for the full amount which had legally expired.
Yes. If any settlement is offered like this, it should be made on a 'without prejudice' basis (together with a full denial of liability in respect of the same) and in full settlement of all liabilities, rights of action and losses + costs in respect of the same.