attempts to match existing exchange offers. When a match is found, the minimum number of units
 between the number of units offered and the units requested are exchanged at a rate matching the lowest buy offer<br>
 A single transaction can match multiple buy offers or none.
 Unlike asset ask order, currency sell is not saved. It's either executed immediately (fully or partially) or not executed
 at all.
 For every match between buyer and seller an exchange record is saved, exchange records can be retrieved 