In addition to what GendolIkari said, you cannot overpay for $0. In a hypothetical example where you have 5 Highways in play (everything costs $5 less, capped at $0 from below), it might be tempting to buy a Stonemason for $0, overpay for $0, and get 2 Highways (which cost $0 at the moment). But that is explicitly forbidden in the rules, you cannot overpay $0.
That means if you have only $5 available before buying Herald, you have 2 options: Don't overpay, or overpay exactly $1. That's why the slider couldn't be moved, $1 was the only legal option (besides not pressing the overpay button at all).
I'll try to recreate the situation and see what happens if your discard has cards (because of course you should have been able to topdeck a card, just not the bought-but-not-yet-gained Herald).
For bug reports like this, it is very helpful if the game number/id is given. It is listed at the top of the game log and in the chat window at the start of the game (it currently has 8 figures and starts with 28...). That allows me and others to look at the situation exactly as it happened, and find maybe the circumstances that the player forgot, or that there is actually a bug that only happens when the stars align just right.