Learn English – “Write an update to people”

They mean two different things.

"Only write the answer" means write it and do nothing else with it. Depending on the context, this might mean "write it, but don't speak it out loud". Or for example if someone said, "Examine this broken automobile part. How would you fix it? Only write the answer", that would mean, "Write how you would fix it, don't try to actually fix it."

"Write only the answer" means to write the answer and nothing else. This would likely mean that you should not write the question on your paper along with your answer, or not include your name, or whatever other information someone might think of writing in addition to their answer.

In general, you would not write out the minus sign in descriptive text.

The company suffered losses in excess of $7 billion.

The company suffered a 2.4 million dollar deficit.

The words "losses" and "deficit" make the minus sign redundant and unnecessary.

The only example I can think of where you might need to be explicit would be a table, where you might be showing both losses and profits. In that case you would probably use the (loss) syntax common in financial docs:

Company X: $2.1M

Company Y: ($1.4M)

Though you could conceivably use a minus sign as well and your point would be made clear:

Company X: $2.1M

Company Y: -$1.4M