Thanks for confirming! 
It’s not possible to do this directly from the sales invoice unless it’s a custom nominal code. So you could create one, for example “Client Travelling”, log all your travelling there and set it to be used in sales invoices, by clicking the cog in the Chart of Accounts Report (Reports >> Chart of Accounts). This will show something like the below, where you have a check box to enable it for sales invoices:
The other option is to use the rebill feature, which will create a new sales invoice for you based on the purchase invoice. If you contra the expenses, it will automatically use the same nominal codes. More info on this here.