Your are saying this because his not an England player, if he was an English player you wont say 100million is small for such a talented striker like Viktor, how much did Manchester city bought Gealish from Aston villa was it not 100m, so Viktor worth 100m too, any club that can pay should pay and get him, I know Arsenal won't pay such amount for him, so Manchester united is at advantage because Amorim can convinced him to join Manchester united with him as his former coach, Manchester united has a chance to sign him than Arsenal. But Manchester united won't spend such amount on him, they will negotiate the best price.
At the end of this season, many changes are going to happen because many big players are already announces about their unavailability for next season which is giving chance to their clubs for going new players currently few big names are in serious discussion for joining big English Players like Osimhen and Viktor.
Situation is not clear because right now talking anything is too early Viktor is surely good talent, but I am not expecting any club will go with amount like this because Sporting tagging 100 Million Euro which is huge hopefully near the end changes will happen if Sporting serious to sell him because Arsenal is having limited budget which will never allow them to spend amount like this on a player which is risk because performing here in English conditions are usually not easy for many players those are doing good in other leagues.