As its a RAW file open it in ACR. At the bottom of the preview window you'll see the file details. Where it states 240 dpi click on that part of the data and you'll get the opertunity to select 300 dpi.
Now open the image. Go to Image>image size and check the dimensions. You'll probably find they are not as requested. Now set the height to the 303 mm they want and check that the width exceeds the length. Select bicubic as the method of resizing and click OK.
Now you've got an image that is slightly too long. Choose the crop tool, and in the dialogue box at the top of the screen enter the dimensions you want. This locks the crop tool to those sizes. Now crop as required. save as Tiff or jpeg and jobs done.
As far as price is concerned that's up to you to negotiate. Ask what price are they prepared to pay, and if it acceptable then OK. But have a figure in mind during the conversation. The price will depend on what their budget is. That will depend on the publication. Also find out what the terms are. Is it payment on publication and if so when will it be published, or something like 60 days after submission. If you can ,get a written order from them stating Terms and conditions. If not include a note stating the T&C's you agreed over the phone with them.
Also make sure that you have a copyright statement attached to your submission. You will need to let them know what rights they have. For example for publication in that publication only, not for a free use for any publication they may care to use it in.
