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There is a reason currency symbols like $ and £ exist and are placed at the front of the number, not at the end.
The symbol and its presence at the front of the number is an anti-fraud token.
The anti-fraud token at the front of the number, with no space between the token and the number, prevents black hats from modifying bank wiring instructions, invoices, checks, and the like, through the addition or removal of an extra digit at the front of the number.
Fraud and embezzlement have always been concerns in business, and paper records (or photos of paper records) remain both a common part of business and a common vector for fraud, which is why the placement of the anti-fraud currency token remains relevant after all these centuries.
A period (or comma depending on locale) at the back end of the number followed by the count of cents or similar acts as a second anti-fraud token preventing manipulation of the amount through addition or removal of an additional digit on the back end of the number.
We say "one hundred dollars" because the phrase form requires the token at the end on the printed page, but we write $100 (short for $100.00) because the numeric form requires the token at the front on the printed page.
It was also conventional historically in high stakes contexts like contracts and checks to write out the final or total amount a second time in words, again as an anti-fraud token ("in the amount of one hundred dollars"), this time with the word "dollars" at the end of the phrase to prevent fraud where a transfer of "one hundred" becomes a transfer of "one hundred and ninety nine". The phrase form is commonly more difficult to modify on paper and also serves as a checksum to validate against the numeric form of the amount.
123456.789 euros is written as "123.456,79 €"