When you create a new line item on the SA, at that time the price and conditions are taken from your info record and copied into the SA item conditions.
Each time a new release is done (followed by receipt per purchasing process), the price is then taken from the SA item condition.
I dont believe its possible for it to take it from the inforecord directly, or update the SA conditions when the info record is updated. You might consider an enhancement to do this update automatically, or maybe something the MIGO BADI / user exit to take the info record price instead of SA conditions.
Its similar to a PO.... When a new PO is created, it will take the info record conditions, but the receipt information is driven from that PO document... If you changed the info record for that applicable PO and did a GR, it still takes it from the PO , not the info record... The next PO created will have the new conditions etc.