You could handle it in the microflow that handles the REST call and parse the string value into a decimal value by using parseDecimal() action. But I would recommend to make sure the string always contains the correctly formatted format or to build a propper handling for the case when it is not possible to parse it correct. Also, you can take a look for fruther details here on the Mendix doc : https://docs.mendix.com/refguide/string-function-calls
If you are using an import mapping based on a json structure, it should be possible to just remove the quotation marks in your json structure. Thy system will now interprete the string as decimal. You only need to make sure that you really only get decimals. Otherwise, importing will fail.
Try Import mapping/export mapping to get/post the data. It will pass your data in JSON format and you can pass the value in desire datatype.
you can take a look for fruther details here on the Mendix doc : https://docs.mendix.com/refguide/call-rest-action
Hope this will help you