If I understand correctly, (a) under common law, theft requires an intent to permanently deprive the owner, and (b) legal tender only applies to debts, not purchases. B is the reason why stores do not have to accept cash. However, if I take goods out of the store without paying, but with the intent to pay later, I have not stolen the goods. By taking them, however, I now owe their value to the store. At this point, it is a debt; can I now insist on paying in cash even if the store does not want to take it?
This is a good example of the life of the law being experience and not reason.
While there is a logical argument that this isn't theft, in reality, this conduct would universally be considered an open and shut case of shoplifting and anyone who tried this would surely be convicted of a crime with consequences far more severe than creating a tort debt for conversion of the property.
Also, you do intend to permanently deprive the store of its property. The fact that you intend to remedy that by paying for it doesn't change that. You aren't borrowing the property with an intent of returning it.
The store is only suggesting a price, that's called "an offer to treat". You can't demand the store honour the suggested price, it is only when you make an offer to the store, with money, and they accept your offer to them, that the contract for sale is completed. You do not get to take an item, and then unilaterally decide what the value of the item is.
You taking an article from a shop and the shop taking money from you are two separate events that may be connected by an agreement to make a sale, a contract that is proven to exist by the receipt.
You cannot force, unless very special and specific circumstances warrant it, someone into a contract. The shop is not obligated to sell anything to anyone (unless specific legislation says so).
Your claim that taking an article out of a shop with the intent to pay later is not theft may be a thesis you can bring up in court but you can still be prosecuted for shoplifting. Whether the fact-finder (most probably a jury) believes your declaration of intent is up to debate; as of this posting, courts can't read minds (and let's hope it stays like this).
These claims, should you make them outside court, may be dismissed as hearsay because they may be considered self-serving statements.
A self-serving declaration refers to a statement made by a party in his/ her own interest at some place and time out of court. It does not include testimony which the party gives as a witness at the trial.
Self-serving declarations are hearsay. “The purpose of the rule rendering hearsay evidence inadmissible is to prevent manufacturing evidence; hence self-serving declarations are excluded by courts”. [Hill v. Talbert, 210 Ark. 866 (Ark. 1946)] ref
What is shoplifting anyway? (emphasis mine)
Shoplifting is generally defined as the unauthorized removal of merchandise from a store without paying for it, or intentionally paying less for an item than its sale price. However, shoplifting can include carrying, hiding, concealing, or otherwise manipulating merchandise with the intent of taking it or paying less for it.
In many states, shoplifting is considered to be a form of larceny and may be prosecuted as such. Other states differentiate between the crimes of shoplifting and general theft for purposes of charging and sentencing, and treat shoplifting less severely than other theft offenses (such as an infraction rather than a misdemeanor). [ref]
In california, you don't even need to leave the shop, just enter it and behave in a way that raises suspicion.
459.5. (a) Notwithstanding Section 459, shoplifting is defined as entering a commercial establishment with intent to commit larceny while that establishment is open during regular business hours, where the value of the property that is taken or intended to be taken does not exceed nine hundred fifty dollars ($950). [ref]
Again the pesky "intent" but any search of previous trials of shoplifting (not many recent ones, I reckon) may show you how prosecution determines intent.