So the long short of it is that having two fee schedules brings up many grey and not so grey areas for compliance and is often more trouble than it's worth as it also makes it harder to reconcile/track/post payments vs. charges. Providers are allowed to offer a prompt-pay discount, however there are certain guidelines for what qualifies.
It is hard to pinpoint a specific law or regulation that says that you should charge all payers (including cash/self-pay patients) the same rate for a specific service, however, there are quite a few things that I think that together would be a red flag for changing prices inconsistently for the same service. The best and most compliant process is as you indicated has been done historically, having one start date in which the fee increases are in effect across the board. Below are a few of the issues that come to mind that would effect this practice.
Things to consider for commercial or government payers:
1. Your contract with each payer - many times the terms of these dictate the amount of your fee increase and how often you implement them. Some contracts may require the provider give prior notice of the fee increases in writing. This could potentially be a compliance issue with the scenario you provided.
2. False Claims Acts - Programs like Medicaid and Medicare are typically the payer of last resort (and the payer of least reimbursement). False Claims Acts can come into play when such programs are billed more than other payers. There are some exceptions, such as a prompt-pay discount for self-pay patients. This can come into play even if the "payer" is not an insurance company, an example of this is Quest laboratories $241 million dollar settlement in California for falsely representing that their fees charged to Medi-Cal were less than or equal to the best price available for other payers (including other physicians, clinics, hospitals, etc.). The discounts available for those other parties were also considered a kickback under the anti-kickback statute, so those violations were also calculated into the settlement amount. (
http://www.mwe.com/publications/uniEntity.aspx?xpST=PublicationDetail&pub=6663)
3. Legal implications of "cash discounts" - You mention that it seems only cash-pay patients are receiving the old price (which I would interpret as equating a discount). Here are a couple of articles which discuss the legal minefield of offering "cash discounts" to patients.
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http://www.healio.com/ophthalmology...al-implications-of-cash-discounts-to-patients
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http://m.arentfox.com/newsroom/alerts/cash-discounts-patients-navigating-legal-minefield
Now, for your scenario you might not feel the description of "cash discount" in these instances would apply, however keep in mind that if a patient presents to your provider as cash or self-pay and ends up recouping reimbursement from insurance (for example, if they come to you out of network and bill insurance for the visit, or if they are retro-eligible for Medicare/Medicaid) this practice could cause problems.
This is just the surface of some of the issues to consider that might arise from having two charge masters or fee schedules being billed to different payers.
On the flip side, providers are allowed to offer prompt-pay discounts, and the lower charge being offered to the cash patients may qualify. The specifics on this can be found here:
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https://www.foley.com/oig-approves-...ram-for-inpatients-and-outpatients-02-15-2008
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https://www.foley.com/files/Publica...-4647-a78b-76e3adbac1e0/084394_LNAlert_HC.pdf
If you want to do further research, I would suggest using some of these search terms: "false claim act" along with "cash discount", "differential charging", or "prompt pay discount" to get you started.
Hope this information is helpful to get you started in getting the answers you need.