While not necessarily sounding like a hidden surcharge, it does sound a little strange/fishy to offer a discount for cash payment on a B2B service. As a service business, you are perceived to be "above" that sort of thing. Personally, my expectation would be of a somewhat inferior service, altho' I might not be able to resist a bargain!

However, it's perfectly acceptable to offer a discount for volume on B2B services e.g.

Buy Package One and get Package Two at a reduced rate. Or buy all three Packages for 20% off the total.

And to assist cashflow, it's perfectly acceptable to offer a discount for prompt payment e.g.

Take 5% discount off the total shown, if paid by the 15th of the month of invoice.

You would still want me to pay by EFT.

On the other hand, it's quite acceptable to have a cash only price if for a retail product. The reason being, quality of the product is a given. I can compare like with like over two or more suppliers. Of course, such offers attract bargain hunters, who are seldom repeat purchasers.

Do you include EFT as cash equivalent, or are you generally referring to payments by card? If EFT does the job, why not cease to offer credit card terms?

The point I'm making with retail bargain hunters is that their only priority is the cheapest price. They will buy the product on (temporary) special over all other products fulfilling the same function. When the promotion period is over, the customer will continue to seek out the cheapest in the category. If however, the product positioning is to always be the cheapest, it may be coincidental that the product is bought more often. 

For services, the opposite is true. A service provider is generally selling on every other factor besides price – reputation, higher quality, increased return on investment, better fit-for-purpose, a greater degree of customisation etc. All these are designed with repeat purchases in mind. The positioning is completely different.

A services firm wanting to appeal to price-conscious buyers would offer a 'one-size-fits-all' commodity product requiring no customisation that can be delivered with minimum effort and an assured outcome. For some firms, that product will be a good fit with other 'productised' services. For others, it would create a perception problem.
 

Hey Kaycee! I think in today's environment where most businesses are going cashless, it may strike odd that there is a cash only option.

That said, my answer will depend on what you are selling and your brand strategy.

In most scenarios I would suggest avoiding incentivising cash payments as it may negatively  impact credibility. To me, that is worth more than the additional margin.

@Benjamin Lai , talking about pricing challenges, this one reminded me of you ;)