Is the plaintiff considered sinful and a liar for swearing to the seller that she did not possess additional money beyond what was in her purse, while she had other money elsewhere, and what is the ruling on her going to complete the price of the item despite the seller forgiving her?

If, when you swore to the seller that you had no money other than what was in your wallet, you meant that you had no other money on your person at that moment, and this was true, then there is no breaking of the oath (hanath) upon you, nor any expiation (kaffarah). However, if you intended an absolute denial and you possessed other money and knew that, then you lied and committed a "ghamus oath" (a false oath sworn deliberately to usurp a right). You must repent, and according to the majority of scholars, no expiation for an oath is required for it. The sale is valid as long as the seller agreed to the stipulated price, and your false statement does not invalidate the sale or obligate anything further from you, because what matters is consent at the time of the contract. Remember that honesty in buying and selling brings blessings, while lying obliterates them.