While officially there is only one Chinese written language, there are many spoken dialects. This written language is referred to as 白话 (báihuà) and is used across China. The spoken languages (i.e. dialects) can differ significantly from 白话 - both in vocabulary and in grammar. Thus, spoken Cantonese differs substantially from 白话. It is key here to realize that the spoken Cantonese language and the official written language (白话) do not correspond to each other!
There is a trend in certain dialects to actually write down the language as it is spoken, which means the syntax, vocabulary and characters can differ from those found in 白话. For example, the site Canton 168 – 廣東一路發 gives the following example:
Mandarin: 我們明天可以去香港 Cantonese: 我聼日可以去香港
This trend for using written Cantonese is most pronounced in Hong Kong. Because this is not an official written language, people themselves must agree on what characters to use in written Cantonese. Over time, certain characters become more popular than others and thus become the de facto character to be used in a specific word. For 做番 and 做返, 番 and 返 are two competing characters, with some people preferring one and some the other.
做番 and 做返 thus represent written Cantonese, not 白话, and are two different ways to write the same word! As Zannjaminderson pointed out in a comment above, in 白话 the term would be rendered as 做回.